This invention relates generally to carriage drive systems for printing and scanning devices, and more particularly, to an apparatus and method for varying belt tension in a carriage scanning system.
In inkjet printing systems and document scanning systems a carriage is moved relative to a media to either print or scan the media. In an inkjet printing system, the carriage carries an inkjet pen which ejects ink drops onto the media as the media is moved along a media path. In a document scanning system the carriage carries an optical sensor which detects ink markings or characters on the media as the carriage moves relative to the media. Conventionally, the carriage is driven back and forth by a timing belt. The timing belt is driven by a pulley on a motor shaft, and is kept in tension by an idler spring. The maximum acceleration of the carriage in a timing belt system is a function of belt tension and carriage mass. Beyond the maximum acceleration the stability of the carriage decreases. The belt tension is controlled by the idler spring. For large carriages or higher acceleration rates, the desired belt tension for accurate control is larger than for smaller carriages and lower acceleration rates. If the belt tension is raised, however, the load on the drive motor increases, which in turn can shorten the useful life of the motor. Accordingly, there is a need for a drive belt system which can operate at increasing acceleration or carry larger masses without shortening the useful life of a given drive motor.
To achieve accurate printing or scanning, it is important to know or maintain an accurate positional relationship between the carriage and the media. In inkjet printing it is important that the carriage scan the inkjet pen smoothly across the media with minimum vibration so that ink dots can be accurately placed. Conventional inkjet printers print 300 dots per inch or 600 dots per inch. In addition, printers which print at 1200 dots per inch are being sought. As the number of dots per inch increases, the dot size has decreased. Precise dot positioning of the smaller dots at increasing dot density leads to higher quality images. In particular, such positioning of colored dots is leading to near photographic image quality. One challenge in striving to achieve such improved image quality is the adverse impact of carriage vibrations. FIG. 1 shows two overlapping circles 12 of a common size. Each circle 12 represents an inkjet printing dot of a first size. Such size is largely exaggerated here for purposes of illustration. FIG. 2 shows two overlapping circles 14 having a common second size which is smaller than the first size. Again, each circle 14 represents an inkjet printing dot of a second size, and such size is largely exaggerated for purposes of illustration. In each example, the dots 12 and dots 14 overlap by a common percentage of their respective diameters (e.g., 20%). The absolute distance of overlap is larger for the larger dots 12 than for the dots 14. The overlap of dots 12 is a distance x. The overlap of dots 14 is a distance y. For purposes of illustration, assume that dots 14 are half the size of dots 12 and that y=0.5x.
Consider now a situation where the carriage vibrates during printing along an axis 16. If the vibration amplitude along axis 16 is much smaller than the distance x, then the impact of the vibration will not adversely impact the dot placement accuracy, and thus will not adversely impact the image quality. As the vibration amplitude along axis 16 approaches the distance x, however, more white space occurs on the media in the vicinity of the dots 12 intersection. Taken over an entire image, the effect appears as a banding of lighter and darker areas of the image. FIG. 3 shows an exemplary image 18 exhibiting such banding.
Given the same amount of vibration amplitude, the impact to an image formed of the smaller dots 14 is more adverse than to an image formed with the dots 12. For example, a vibration amplitude of 0.25x may be acceptable for printing using dots 12. The same vibration amplitude equals 0.5y and may cause unacceptable banding when printing with the dots 14. Such bands occur within an image at the frequency of vibration of the carriage along the axis 16. In general, the smaller dot size and higher resolution of advancing ink jet printers require more accurate placement of dots to achieve expected image quality improvements.
Any vibrations displacing the carriage relative to the media can potentially reduce printing/scanning accuracy. Typical sources of vibration are external vibrations which move the whole printer or scanner, and internal sources which are coupled to the carriage or media. This invention is directed toward internal vibrations which are coupled to the carriage.